Hydrogeological investigations and numerical simulation of groundwater flow in the karstic aquifer of northwestern Yucatan, Mexico

A freshwater lens underlain by saltwater exists beneath the entire northern Yucatan karst plain, Mexico. Water levels recorded in this plain, during the period June 1987 April, 1989, were used to map the water table, identify inland hydrogeologic boundaries, and estimate the thickness of the freshwater lens using the Ghyben-Herzberg relation. The water table under most of northwestern Yucatan is less than two meters above mean sea level. Normal dry/wet seasonal variations during the period of study were about 0.5 m. The Ghyben-Herzberg estimated thickness of the freshwater lens ranges from a low of 18 m near the coast to more than 80 m in the southeastern section of the study area. A quasi-three dimensional finite-difference model, in which freshwater and saltwater flow are separated by a sharp interface, was used to simulate groundwater flow in the study area. The conceptual model that best reproduces measured water levels is a two-layer system with a deep layer of high hydraulic conductivity (1 m/s) overlain by a layer of lower hydraulic conductivity (0.1 m/s). The layer of high hydraulic conductivity corresponds to a zone where chemical erosion probably occurred between 35 and 120 thousand years ago, when sea level oscillated around a level about 50 m below the present level. Simulation of a breach in a shallow coastal aquitard showed that proposed construction, which would cause such a breach, could result in a substantial reduction of the water resources of the Yucatan plain. INTRODUCTION The aquifer in northwestern Yucatan contains a freshwater lens that floats above a denser saline water wedge that penetrates more than 40 km inland (Back and Hanshaw, 1970; Durazo et al., 1980; Back and Lesser, 1981; Gaona et al., 1985; Perry et al., 1989). Recently, it’s been shown to penetrate more than 90 km (Perry et al., 1995; Steinich and Marín, 1996). The aquifer, which is unconfined except for a narrow band along the coast (Perry et al., 1989), is the sole freshwater